The purpose of the present proposal is to study a puzzling phenomenon that detrimentally impairs some human patients with unilateral brain lesions, usually to the right posterior cortex. These patients tend to ignore or inattend visual and/or auditory and/or tactile stimuli to the side contralateral to the lesion when simultaneously bilaterally stimulated. This aspect of unilateral neglect has not been observed under conditions of unilateral stimulation which portrays the puzzling feature of this phenomenon. This phenomena will be studied under precise experimental control following an operant methodology. Major emphasis will be devoted in attempting to retrain these patients to correctly discriminate between unilateral and bilateral simultaneous sensory stimulation. Three different and unique experimental paradigms will be followed in this attempt. It is hoped that this research will provide additional theoretical as well as methodological information in trying to understand this startling response pattern. Furthermore, if the retraining objectives can be readied, they will provide explicit implications for treatment intervention of these patients. This research project has been designed to achieve these goals within an economical period of time by maximizing the presently available resources and equipment.